A pneumatically driven ram boring machine having an impact tip mounted in a cylindrical housing and a percussion piston reciprocating in the housing is known from German patent specification 21 57 259. The automatic piston of this apparatus exerts periodic ramming blows on the movable impact tip. Under the influence of the ramming blows the impact tip, which is supported on the machine housing through a compression spring, oscillates and moves into the ground and, when its stroke is exhausted, finally draws the housing after it.
Such ram boring machines serve primarily to lay service lines such as, for example, electricity or telephone lines or cables under roadways or sidewalks without the need to tear up the surface of the roadway or sidewalk, As the ram boring machine moves through the ground it forces the soil aside and leaves behind it a passage into which a service line can be drawn at the same or some subsequent item. Such a ram boring machine can therefore be provided at its rear end with a connection for fastening a follower pipe, or alternatively a pipe to be laid can be pulled into the passage in the ground by the ram boring machine using a tow rope.
It is known from German patent specification 28 24 915 to use such ram boring machines for the destructive replacement of old lines, for example comprising iron or ceramic pipes. For this purpose the impact tip of the device is provided with cutting or impact knives that project radially outwards. As the impact tip moves forward the cutting edges burst the walls of the old line, though the rubble remains in the ground.
Although the impact tip also breaks up or pushes aside stones and other obstructions in the ground, it is not always possible to avoid the ram boring machine departing from the desired direction when it encounters an obstruction. Such deviations in course are only of minor importance when the bore is short, for example when crossing beneath streets and railway embankments, but they lead to considerable difficulties when precise, directionally accurate boring over long distances is needed. To enable the direction of advance of the ram boring machine to be controlled over bores extending for long distances target pits are therefore dug at suitable distances apart.